Electrochromic mirrors, also referred to as electrochromic mirrors for short, have the property that they change their reflectivity under the influence of an electrical voltage. They are used preferably in the car industry, in particular as automatically dipping rearview mirrors in motor vehicles.
Such automatically dippable rearview mirrors improve traffic safety when driving at night by reducing eye strain, and help to avoid accidents.
When the driver is in a dark environment, his pupil operates like a camera shutter which is in a wide-open setting, in order to supply the optic nerve with as many light signals as possible. In this state, the eye reacts extremely sensitively to a sudden incidence of harsh light. The light which conventional rearview mirrors reflect from the headlamps of vehicles which are overtaking or following dazzles the driver to such an extent that his ability to see is severely reduced for an instant. For fractions of a second, the driver is virtually blind as a result of the optical after-effects of the blinding light and his reaction time to obstacles which are becoming visible on the carriageway in front of him is increased by more than a second.
In systematic braking distance measurements, this so-called Troxler effect led, fore example, to a doubling of the braking distance of a passenger car under test under night driving conditions at 100 km/h on a dry carriageway.
All persons driving at night are affected by this irrespective of their sex, age or eye colour. This effect constitutes, for night driving, a risk factor which is hardly adequately compensated by conventional, mechanically adjustable anti-dazzle interior mirrors. When the interior mirror is tilted, the reflection is reduced from approximately 90% to 4%. The low residual reflection of the tilted interior mirror reduces the dazzling effect but still makes it possible only to recognize the headlamps and not the contours of the vehicles which are overtaking or following. In exterior mirrors which are curved in the most common way, such a tilting feature as an anti-dazzle protection for night driving is not provided.
However, this is evidently remedied by equipping motor vehicles with automatically dippable rearview mirrors whose electro-optical sensors detect the risk of dazzling instantaneously and reduce the risk in a strain-relieving fashion for the eye by sliding reduction of the mirror reflection to approximately 10% within a few seconds. When the risk of dazzling is past, the mirror reflection is increased immediately back to the starting value. This automatic interplay of darkening and lightening of the rearview mirror is repeated, without creating any fatigue phenomena, whenever there is a risk of dazzling during the entire service life of the vehicle.
In order to change the reflection behavior of the automatically dippable rearview mirror, the electrochromic behaviour of specific chemicals, which are arranged in front of the actual reflector of the electrochromic mirror, has been used for more than ten years. Some inorganic and organic electrochromic "colouring agents" change their absorption depth if they have an electric potential applied to them via adjoining electrodes.
Such reversible changes in absorption are known in a number of inorganic oxides of thin solid layers of transition metals (for example tungsten), and have been systematically researched. On the other hand, thin liquid crystal cells with organic colouring agents (for example liquid crystals or viologens) have been successfully investigated in this context. For reasons of function and cost, liquid crystal cells with viologens have become adopted for commercially available electrochromic mirrors.
These cells, referred to below as optically active cells, form the heart of the electrochromic mirror. They are typically composed of two plates which are shaped to correspond to the mirror configuration and are preferably made of glass, i.e., a front glass and a rear glass, which are connected to one another spaced apart and sealed off from the surroundings along their circumference. Between the two plates there is the electrochromically active medium, in particular a liquid with viologens. Each plate is provided in each case on the side facing the electrochromic medium with an electrically conductive electrode layer which covers the entire plate surface and to which in each case a connecting wire is attached. On the rear glass there is the "actual" mirrored layer.
If a voltage is applied to the two connecting wires and the associated planar electrode, for example the voltage generated by a light sensor owing to the light of a passenger car travelling behind, the absorption depth of the electrochromic medium arranged in front of the mirror plate, and thus the reflectivity of the optically active cell, changes.
These relationships are the prior art and have been disclosed in numerous documents.
A particular problem with such a typical cell is the formation of mutually insulated contacts between the connecting wires and the associated planar electrode since the planar electrodes lie one on top of the other in congruency, only being separated by a very narrow gap of approximately 0.1 to 0.2 mm.
U.S. Pat. No. 5,151,824 has disclosed how the problem can be solved in such a way that the front and rear glass are arranged opposite with respect to one another by prescribed amounts so that on each glass an exposed zone of the planar electrode is produced which can be used for contacts. On these edge zones there is in each case an elongated contact clamp with resilient contact tongues which embrace the glass with the exposed edge zones of the planar electrodes and to which in each case the connecting wire is soldered.
As a result of the offset of the edges in the known electrochromic mirror, it is a disadvantage that, in the first instance, the edge zone of the mirror is relatively large, which is not desired. The requirements of the car industry prefers electrochromic mirrors which are virtually indistinguishable in dimensions from conventional mirrors. In addition, the spring or clamp contact is very complex and awkward to attach and also can only make contact with the planar electrode in the relatively narrow region. This has a disadvantageous effect on the speed with which the absorption depth of the electrochromic medium changes.
EP 0 434 453 B1 (=U.S. Pat. No. 5,066,112) has disclosed an electrochromic mirror of the generic type which does not have an offset of the plates of the optically effective cell or any spring contacts in the form of clamps in which an additional, conductive contact layer is applied to the planar electrode in the edge zones of the plates, including their end side, the connecting wire then being soldered to the end side.
Such an electrochromic mirror is, in the first instance, very complex to manufacture and, secondly, the end-side contact zone for attaching the connecting wire is very narrow, with the result that the latter can easily tear off and, on the other hand, also only allows a very narrow contact face, which also has negative effects on the speed at which the absorption depth of the electrochromic medium changes.